(i) Field of the Invention
The present invention relates to a method of coating a steel pipe or tube and, more particularly, relates to a method of providing a protective, corrosion-resistant coating of a metal alloy on a carbon or low alloy steel pipe or tube.
(ii) Description of the Related Art
Downhole oil and gas drilling, production and casing tube strings and tools conventionally are fabricated from carbon steels and low-alloys steels which are prone to corrosion and to erosion under hostile subterranean environments. There accordingly is a need for protective surface coatings on such steel components.
Tubing fabricated from nickel base alloys such as UNS N10276 (ASTM E 527/SAE J 1086) typically are used in deep sour gas production wells having severe corrosion problems from the presence of hydrogen sulfide (H2S), carbon dioxide (CO2) and sodium chloride (NaCl) in the environment. UNS N 10276 alloy, one of the so-called corrosion resistant alloys (CRAs), contains chromium, molybdenum and other alloying elements such as tungsten. As the CRAs are expensive, their use is limited to those wells with very severe corrosion problems where alloy steels or stainless steels are not suitable.
There have been many attempts to produce low-cost corrosion-resistant tubular goods by various methods such as coating, cladding or surface welding, as described by L. Smith in the British Corrosion Journal, Vol. 34, No. 4 (1999) pages 247-253. However, to date there is no commercial product available in the market because of the cost and/or the technical difficulties encountered in the aggressive environment of sour gas fields.
Cladding of steel tubes can be done either by mechanically bonding a thin walled UNS N10276 alloy sleeve to a low alloy steel tube or by metallurgically surface welding the sleeve to the tube. Cladding is a well-known process for covering sheet metal and tubular goods and several clad metals utilizing cladding technology based on different manufacturing processes have been proposed. The various manufacturing processes include simple insertion of a corrosion-resistant liner inside a carbon steel tube and sealing the ends by welding; insertion of a corrosion resistant liner into a carbon steel tube, expanding the liner by pressurized fluid and sealing the ends by welding or by brazing a soldering material between inner and outer tubes; explosive bonding of a corrosion resistant inner sleeve to a carbon steel tube; utilizing hot isostatic pressure to bond an inner tube on outer tube; and shrink-fitting through heating and cooling by utilizing the difference in the thermal expansion coefficients of the inner and outer tube materials (inner tube shrinks less than the outer tube creating interference stress at the interface).
Centrifugal casting, described in the U.S. Pat. No. 4,943,489 (1990), is known for producing a composite pipe. This technique involves pouring a carbon steel in the molten state into a rotary mold to form on outer layer, pouring a corrosion resistant material into the mold after the solidification of the outer layer to create an intermediate layer through reaction between the outer layer and the corrosion resistant material, and continuing pouring the corrosion resistant material to form an inner layer. This method creates a three-layer structure: a 3 mm inner layer, a 20-100 micron intermediate layer and a 15 mm outer layer. This foundry-based process is considered complicated and expensive and thickness control is a problem at low ends.
Powder metallurgy based techniques have been also attempted many times to produce internal coatings inside tubes. The methods involve placing appropriate powder with or without a binder on the internals surfaces of the tubes and sintering using laser, electron beam, plasma source or other appropriate heating mechanisms.
Plasma spraying is a technique also used to coat inside of tubular goods. The inherent porosity of the coating limits its use in corrosion-related applications. Laser remelting of the plasma sprayed coatings appears to help minimize the porosity problems. However, coating of internal surfaces of long tubes with small diameter is a key limitation of this technique.
Plasma transferred arc (PTA), as disclosed for example in U.S. Pat. Nos. 4,878,953 and 5,624,717, is a technique used to apply coatings of different compositions and thickness onto conducting substrates. The material is fed in powder or wire form to a torch that generates an arc between a cathode torch and the substrate work-piece. The arc generates plasma in a plasma plume that heats up both the powder or wire and the surface of the substrate, melting them and creating a liquid puddle, which on solidification creates a welded coating. By varying the feed rate of material, the speed of the torch, its distance to the substrate and the current that flows through the arc, it is possible to control thickness, microstructure, density and other properties of the coating (P. Harris and B. L. Smith, Metal Construction 15 (1983) 661-666). The technique has been used in several fields to prevent high temperature corrosion, including surfacing MCrAlYs on top of nickel based superalloys (G. A. Saltzman, P. Sahoo, Proc. IV National Thermal Spray Conference, 1991, pp 541-548), as well as surfacing high-chromium nickel based coatings on exhaust valves and other parts of internal combustion engines cylinders (Danish Patent 165,125, U.S. Pat. No. 5,958,332).
This technique has been proposed for coating internal surfaces of tubular goods used in oil field applications. The excessive coating thickness has been such that the total cost remained high and rendered the process uneconomic in small and medium tube size ranges.
Key limitations of known PTA process are the inability to deposit thin layers due to large waviness of the deposits, necessitating larger machining allowance and hence thick deposits to obtain smooth surfaces. Excess dilution from the substrate on one hand or lack of bonding on the other hand often results in poor coating.
Other coating techniques reported in the literature include physical vapour deposition (PVD), chemical vapour deposition (CVD) and thermal spraying combined with laser remelting. Some of these surface treatments did not go beyond lab scale testing but others extended to full scale field-testing. However, none of these coatings has been fully adopted by the oil and gas industry notwithstanding the continuing need for corrosion-resistant pipe and tubing in oil- and gas-producing wells.
The apparent lack of interest in these surface-engineered clad tubes results from the high cost of applying the coating with respect to solid wall CRA, lack of satisfactory coating performance due to porosity or similar defects in the coating (e.g. titanium nitride coatings by PVD), and complications in designing connectors for clad tubes.
It is accordingly a principal object of the present invention to provide a method for coating long lengths of steel pipe and tubing, particularly carbon and low alloy steels, with an inexpensive, dense, continuous and smooth protective coating substantially free of defects.
It is another object to provide a corrosion-resistant coating within long lengths of steel pipe and tubing suitable for use in the corrosive environments of oil-and-gas producing wells.
A further object of the present invention is the provision of a thin corrosion-resistant coating metallurgically bonded to the interior of pipes and tubes by plasma transferred arc deposition, or by slurry coating or thermal spraying and sintering.
In its broad aspect, the method of the invention of providing a protecting coating on a steel substrate comprises metallurgically bonding a continuous thin coating of a MCrX alloy where M=one of nickel, cobalt, iron or combination thereof and X=one of molybdenum, silicon, tungsten or combination thereof, having about 45 to 91 wt % M, about 9 to 40 wt % chromium and 0 to about 20 wt % Mo, 0 to about 20 wt % Si and 0 to about 10 wt % W, by plasma transferred arc deposition of the coating onto the steel substrate or by slurry coating or thermal spraying and sintering. The steel substrate preferably is a plain carbon or low alloy steel and comprises the inner surface of a pipe or tube. The thin alloy coating has a thickness of 0.1 to 10 mm, preferably 0.5 to 5 mm, and most preferably 0.7 to 3 mm.
A preferred MCrX alloy comprises 55 to 65 wt % Ni, 15 to 25 wt % Cr, 10 to 16 wt % Mo, 1 to 4 wt % W and the balance Fe and incidental impurities. The alloy may additionally contain at least one of up to 5 wt % Cu, B, Ti and Nb, up to 1.0 wt % Y, Zr, Ce and C, up to 2 wt % V, up to 4 wt % Ta and up to 0.8 wt % N.
The preferred method comprises preparing the steel substrate by boring, honing, bright finishing, grit blasting, grinding, chemical pickling or electro-polishing the steel substrate prior to deposition of the coating. The preparation of the tube surface prior to deposition determines coating microstructure with acceptable level of porosity. Pre-heating the steel pipe or tube at a temperature in the range of 100 to 800xc2x0 C., preferably 250 to 600xc2x0 C., is effective to avoid cracking and to enhance wetting and bonding of the coating to the substrate. The coated pipe or tube preferably is heat treated at a temperature in the range of 800 to 1100xc2x0 C. for a time effective to restore pre-coating strength, ductility and toughness of the substrate and is smoothed by boring, honing, extruding, drawing, roll-forming, grit blasting, grinding or electro-polishing. A second thin coating of the MCrX alloy having a thickness of about 0.1 to 1.0 mm deposited by plasma transferred arc onto a first continuous thin layer of the MCrX alloy previously deposited by plasma transferred arc provides a smoother coating.
In accordance with another aspect of the invention, the method comprises providing a protective coating on an inner steel substrate of a carbon or low-alloy steel pipe or tube comprising roughening the steel substrate by wet or dry grit blasting, knurling or abrasive cleaning and depositing by slurry coating or thermal spraying a MCrSiX coating powder on the substrate, where M=one of nickel, cobalt, iron or combination thereof and X=one of molybdenum, boron, tungsten or combination thereof, having about 45 to 91 wt % M, about 9 to 40 wt % chromium, about 0.8 to about 20 wt % Si, 0 to about 20 wt % Mo, preferably about 2 to 10 wt % Mo, 0 to about 8 wt % B, preferably 0.8 to about 5 wt % B, and 0 to about 5 wt % W, preferably about 1 to 4 wt % W, and heat treating the coating at a temperature in the range of 600 to 1200xc2x0 C., preferably in the range of about 950 to 1150xc2x0 C., for sintering and metallurgically bonding the coating to the substrate.
A preferred MCrSiX alloy in which M=one of nickel, cobalt or combination thereof comprises 45 to 84 wt % M, 15 to 30 wt % Cr, 0.8 to 8 wt % Si, 0.8 to 5 wt % B, 0 to 20 wt % Mo, 0 to 10 wt % W and the balance Fe and incidental impurities. The alloy additionally contains at least one of up to 5 wt % Cu, B, Ti and Nb, up to 1.0 wt % Y, Zr, Ce and C, up to 2 wt % V, up to 4 wt % Ta and up to 0.8 wt % N.
Pipe or tube coating produced according to the method of the invention preferably has a length of 5 to 50 feet, preferably 10 to 46 feet, and more preferably 20 to 46 feet. The coating has a thickness of 0.1 to 5 mm, preferably 0.5 to 3.0 mm, has a sound metallurgically bond with the steel substrate, and has a dense microstructure particularly suitable for pipe or tubing used in oil and gas production.